Recently, in optical communications systems, the use of signals subjected to phase modulation, which is excellent for reception sensitivity, to perform wavelength division multiplexing (WDM) with high density at a narrow wavelength interval is raising demand for increases in transmission capacity. Phase modulation schemes include differential phase shift keying (DPSK) and differential quadrature PSK.
In optical fibers having nonlinear effects, signal light transmitted by wavelength division multiplexing is affected by cross phase modulation from the signal light of an adjacent wavelength channel (see, for example, Japanese Laid-Open Patent Publication No. 2010-028470). The effects of cross phase modulation become large due to raised output of transmission path input power, increases in transmission distance and relaying, narrowing of the wavelength intervals among adjacent wavelengths.
Further, when phase modulated signal light is affected by cross phase modulation and the wavelength varies, since the demodulation accuracy at the receiving side drops, communication quality also drops. In particular, for high density multiplexing systems that densely multiplex wavelength intervals to increase frequency utilization efficiency, since frequency variations caused by cross phase modulation is great, drops in the quality of communication via phase modulated signal light are also great. Further, technology is known that reduces the effect of cross phase modulation, by flattening the waveform of signal light to be multiplexed (see, for example, Japanese Laid-Open Patent Publication No. 2009-213160).
However, the effect of cross phase modulation varies according to the power and wavelength interval states of the signal light to be wavelength division multiplexed. Consequently, problems arise with the conventional technologies above in that an appropriate reduction of the effect of the cross phase modulation is difficult and communication quality cannot be improved.